You should welcome new users!
Going from lurker to user is an enormous risk that the community should recognize. It's easy to overlook that someone has taken that risk and, instead, focus on what the user's done wrong in the question or answer. So, if a user hasn't already been welcomed to the site, welcome that new user before jumping in to critique the user's answer or question! There's no privilege associated with welcoming a new user, and the more current users that are nice and welcoming to a new user, the better the site will get.
How I do it
I try to do this often, and I try to make the experience positive. I tend to use a welcome message on a question that's like this:
Welcome to the site! Take the [tour]
! [Then insert your own version of Good question or This question could be improved by…, or The site doesn't handle this kind of question or whatever, omitting brackets.] Thank you for participating and have fun.
(I read a study that exclamation points in text messages make the writer sound more sincere, so I end the first sentence with one. Also, putting [tour]
in brackets in a comment automatically links to the site's tour page. Finally, to avoid an accusatory tone, I try not to use the word you in the bracketed portion at all, instead aiming for This question could be better were it edited to include… or something.)
And my welcome message on an answer tends to look like this:
Welcome to the site! Take the [tour]
! [Then insert your own version of Good answer or This answer could be improved by…, or Unfortunately, this isn't really an answer to the question because… or whatever, omitting brackets.] Thank you for participating and for helping [or trying to help if the answer wasn't appropriate, again omitting brackets] strangers.
(See above.) I view each welcome message's last sentence as really important in recognizing the user's transition from just looking around to putting words on the Internet for all to read. However, there's no set format, and if come up with your own welcoming message, that's totally okay.